The Prince of Wales and Duchess of Cornwall yesterday endured a "nightmare" emergency controlled landing in their helicopter on the way to the Telegraph Hay Festival, before battling through rain and hail to meet loyal supporters.

The Prince and Duchess had been travelling from central London to Hay-on-Wye, Wales, to meet local business owners and support literacy programmes at the festival.

Around 15 minutes into their journey, a "technical problem" believed to be with the rear rotor blade of the helicopter forced their pilot to divert to the nearest airfield.

The aircraft, which was carrying around seven people, made an urgent controlled landing – later described as "hairy" – at Denham Aerodrome, Buckinghamshire.

Despite the frightening experience, the Royal couple then insisted they were able to continue the long journey by car to avoid disappointing visitors to Hay.

They endured a lengthy car journey of around four hours, before arriving in Hay-on-Wye nearly three hours behind schedule.

In heavy rain and hailstones, the Prince and Duchess immediately spoke to local businesses owners and apologised for keeping them waiting.

They were greeted by several hundred well-wishers, including classes of schoolchildren who were seeing members of the Royal family for the first time.

Resident Judith Rowley, 53, told the Telegraph she had asked the Prince about the trouble with his helicopter.

She said: “I shook his hand and said ‘I’m sorry about your helicopter breaking down’. ‘I know, I know,’ he replied. ‘It was a nightmare.’

"Then Camilla said, ‘I’m so sorry we’re four-and-a-half hours late'."

One pensioner, who preferred not to be named, added: "They were unflappable despite what they went through.

"If anyone else had gone through what they did they would have cancelled their day."

A spokesman for the royal couple confirmed: "The pilot carried out a controlled emergency landing after diverting to the airport."

Another source added: "It was quite a hairy experience."

After speaking with market stall holders yesterday afternoon, the Prince and Duchess then spent time in a country store, book shop and Hay Castle.

Sian Ward, 31, who is pregnant and will soon take over family run business FW Golesworthy & Sons, said: “I've never met any royalty before, and I was really surprised about by how down to earth they were.

"They asked me how long until my baby’s due. I told them five weeks.

"They were saying how nice it was to see everyone out, and hoped that we didn’t mind waiting in the rain."

Local Margaret Rankin added: “We’re both grandmothers, and Camilla talked about how wonderful it was to have grandchildren. They’re both just so lovely – we’re fanatical royalists.”

As hail gave way to sporadic showers, the Prince and Duchess then attended the Telegraph Hay Festival to launch a literacy scheme.

While the Prince of Wales visited local farmers, the Duchess of Cornwall heard schoolchildren read and spoke to ex-offenders about a successful programme of literacy in prison.

Penny Chantler, a farmer with 300 sheep who lost dozens of lambs this year during the cold and wet spring, told the Telegraph the Prince seemed "genuinely concerned" about the situation.

Hywel Jones, 45, runs the shop Authentically Welsh with his wife, Sue Howells said: “He wanted to know how we were doing, business-wise – we only opened six weeks ago. But it was a caring thing to ask. He’s a great ambassador for agriculture, and a great ambassador for Wales.”

Alex Gooch, an artisanal baker, added: "It’s great that there’s someone to speak out about food and farming. So many influential people in his position sit on the fence, but he says what he thinks.”

After receiving gifts of books from schoolchildren, the Duchess of Cambridge also attended the Telegraph Hay Festival bookshop where she launched this summer's Reading Agency campaign.

The independent charity has linked up with the annual festival to feature a special Summer Reading Challenge. A handful of bestselling children's authors, including Cressida Cowell, Korky Paul and Lauren Child, also attended.

The campaign aims to reach 750,000 four to 11-year-olds and inspire them to read six books during the long summer break.

The Duchess then went on to speak to two former offenders from the Hay-in-the-Parc scheme, which sees authors visiting Parc prison, Bridgend, to help with their rehabilitation.

After the festival visit, the royal couple parted and the Prince of Wales went on alone to Cardiff to attend the Welsh National Opera's opening new production of Wagner's Lohengrin.

The Telegraph Hay Festival, which opened yesterday, will now continue until June 2nd.

You can download a free digital version of The Hayly Telegraph, the daily paper of the Hay Festival, each day of the festival from the iBookstore. Visit iTunes.com/HayFestival or search 'Hayly Telegraph' on the iBookstore.